This study was designed to show the effect of boiling on the antihypertensive and antioxidant activities of onion in N^G-nitro-L-arginine methyl ester (L-NAME) inducedhypertensive rats and spontaneously hypertensive rats (SHR). Male 6-wk-old Sprague-Dawley rats were made hypertensive by being given distilled water containing L-NAME at a dose of 50mg/kg BW/d. These rats were fed diets containing raw or boiled onion at a concentration of 5%. Raw onion significantly reduced the increase in systolic blood pressure in both LNAME induced-hypertensive rats and SHR, and inhibited the increase in thiobarbituric acidreactive substances (TBARS) and conjugated dienes in the plasma and tissues of SHR. The antihypertensive effect of boiled onion was not found, and the antioxidant activity of it was much weaker than that of raw onion. The excretion of nitric oxide metabolites (NO_x) in urine was enhanced by raw onion in both L-NAME induced-hypertensive rats and SHR, and was enhanced by boiled onion only in SHR. In conclusion, our results suggested that the antihypertensive activity of onion disappeared during boiling, and the disappear of the antihypertensive activity of raw onion after boiling might come, in part, from a decrease of the antioxidative activity of onion, with a consequent reduction in the saving of nitric oxide (NO).